Abstract
Determining the sound velocity of iron under extreme thermodynamic conditions is essential for a proper interpretation of seismic observations of the Earth’s core but is experimentally challenging. Here, we review techniques and methodologies used to measure sound velocities in metals at megabar pressures, with specific focus on the compressional sound velocity of hexagonal close-packed iron. A critical comparison of literature results, coherently analyzed using consistent metrology (pressure scale, equation of state), allows us to propose reference relations for the pressure and density dependence of the compressional velocity of hexagonal close-packed iron at ambient temperature. This provides a key base line upon which to add complexity, including high-temperature effects, pre-melting effects, effects of nickel and/or light element incorporation, necessary for an accurate comparison with seismic models, and ultimately to constrain Earth’s inner core composition.
Highlights
Determining the sound velocity of iron under extreme thermodynamic conditions is essential for a proper interpretation of seismic observations of the Earth’s core but is experimentally challenging
Antonangeli and Ohtani Progress in Earth and Planetary Science (2015) 2:3 sets, even including those based on indirect determinations, are generally limited to pressures of the order of 100 to 200 GPa at ambient temperature, and very few results exist at simultaneous high pressure and temperature
Models based on linear extrapolations of VP-ρ relation argue for about 2 wt% silicon alloyed with iron in the inner core (Badro et al, 2007; Antonangeli et al, 2010), while a recent model using a power law for the VP-ρ extrapolation leads to 8 wt% silicon (Mao et al, 2012)
Summary
Determining the sound velocity of iron under extreme thermodynamic conditions is essential for a proper interpretation of seismic observations of the Earth’s core but is experimentally challenging. Literature data will be examined and compared to propose a reference for the pressure and density evolution of compressional sound velocity of hexagonal closed-packed (hcp) iron, the polymorph of iron observed to be stable at Earth’s inner core conditions (Tateno et al, 2010).
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